1. Field of the Invention
The present invention relates to a wafer polishing apparatus, and more particularly to a method and device for measuring the thickness of a polishing pad of a wafer polishing apparatus.
2. Description of the Related Art
In recent years, the micromachining of an IC has advanced and IC patterns have been formed over multiple layers. However, there is no way to avoid that a certain degree of unevenness is generated on the surface of a layer on which patterns are formed. In the past, patterns of the next layer have been formed as they are. However, if the number of layers is increased, the unevenness is significantly generated on the surface. Therefore, it was difficult to form good patterns. Further, since the depth of focus of an exposure device is reduced due to the miniaturization of patterns, it is difficult to transfer good patterns due to some unevenness generated on the surface. For this reason, in recent years, the surface of the layer is flattened by polishing the surface after the formation of patterns, and then patterns of the next layer are formed. A wafer polishing apparatus (CMP apparatus) using chemical mechanical polishing (CMP) is used to flatten a wafer by polishing the wafer in the process for forming the IC patterns as described above.
In general, the CMP apparatus includes a polishing surface plate that polishes a wafer, and a polishing head that holds the wafer. The CMP apparatus polishes the wafer by pressing the wafer, which is held by the polishing head, against the polishing surface plate and rotating the wafer and the polishing surface plate while supplying abrasive (slurry) between the wafer and the polishing surface plate.
Here, since a polishing pad is attached to the surface of the polishing surface plate that polishes the wafer, the wafer is polished while being pressed against the polishing pad. However, since the polishing amount of the polishing pad is decreased by the clogging of the surface of the polishing pad, the dressing of the polishing pad is performed in the CMP apparatus whenever one wafer is polished or while the wafer is polished.
Meanwhile, since the surface of the polishing pad is polished by the dressing little by little, the surface profile of the polishing pad is changed and the flatness thereof gradually deteriorates. When the wafer is polished using such a polishing pad, there is a drawback that the wafer may not be flattened with high accuracy.
Accordingly, in the past, during the dressing, an operator measures the straightness of the surface of the polishing pad by a straightness measuring device and adjusts the dressing in consideration of the polishing amount obtained from the result of the measurement. Alternatively, in the case of a polishing pad with grooves, an operator measures the depth of the groove and adjusts the dressing in consideration of the polishing amount obtained from the result of the measurement.
However, in the method in which an operator manually measures the straightness of the polishing pad as in the past, there are problems in that a lot of time is required for the measurement of the straightness of the polishing pad and efficiency is poor. Further, in the case of the method in which an operator measures the depth of the groove, there are problems in that it is not possible to accurately measure the depth of the groove and it is difficult to perform dressing so as to flatten the polishing pad.
In order to solve the above-mentioned problems, as a technique in the related art, there is disclosed a technique for obtaining polishing conditions and dressing conditions based on the profile of the surface of the polishing pad that is measured by a contact type or non-contact type pad shape measuring device (see Japanese Patent Application Laid-Open Nos. 2000-249009, 2002-270556, and 2002-337046).
However, when the dressing of the polishing pad is performed, the polishing pad becomes thin. For this reason, when the surface of the polishing pad approaches the surface plate, there is a concern that the thin portion may adversely affect the polishing of the wafer. Further, if the surface plate is exposed to the surface of the pad, the wafer to be polished is damaged. Meanwhile, after a long-term use, the surface plate, which is a base of the polishing pad, is also deformed by heat or the like generated by polishing. Accordingly, it is not possible to determine whether the result of the measurement is caused by the polishing pad or the surface plate, only from the measurement of the surface profile of the polishing pad as in the related art. Further, it is difficult to foresee that the surface plate is exposed to the surface of the polishing pad. In order to determine and foresee this, it is necessary to measure the thickness of the polishing pad or the surface profile of the surface plate after separating the polishing pad from the surface plate. However, there is a problem in that a polishing pad cannot be used by being attached to the surface plate again once it is separated from the surface plate.